Introduction of Nervous System

For increasing grade of organisation of body structure and functions, it requires coordination among the systems. Nervous system has evolved for this purpose which also makes body able to detect and receive the sensory stimuli (both external and internal), integrate them, process them to opt for the related response. Therefore, this system has undergone the highest degree of change during the evolution of animals

It acts as rapid and spontaneous system to control the body functions through following ways:

Receiving, decoding and analysing the stimuli and translating them into response.

Conduction of information in different parts of the body.

Retention of this information as memories.

Activation and regulation of various secretions from gland systems.

Sponges have no neurons hence due to lack of coordination their multicellularity has no status of tissue.

In Cnidarians

First to have nerve cells (nonpolar types) forming a nerve net (or plexus) in a single layer.

Impulse conduction is slow and decremental type, useful in producing only localised responses.

In Platyhelminthes

There is first sign of cephalization. Planaria has two ladder-like ventral nerve cords which converge to form a rudimentary brain. Others also have anteriorly situated ganglia and nerve cords running along the body.

In Annelids

First to show actual centralisation of the nervous system with a nerve ring in anteriormost segment and a single ventral nerve cord with segmental ganglia. Nerves arise from these ganglia innervate the nearby tissue/organs.

Cephalization is related to the unidirectional mode of locomotion which also assists in feeding. Head comes first in contact with new environment hence it contains all the sensory structures necessary to detect stimuli associated with these situations.

The nerve ring represents a pair of cerebral ganglia fused above the pharynx and linked to the ventral nerve cord by circumpharyngeal connective.

In Cockroach

The basic plan is typical annelidan type distinct as central and peripheral nervous systems (CNS and PNS).

In CNS six cephalic segmental ganglia form the supra-oesophageal or cerebral ganglia (brain) and sub-oesophageal ganglia connected by circumoesophageal commissure and a ventral nerve cord with 9 segmental (3 thoracic and 6 abdominal) ganglia. The 6th abdominal ganglion is large and represents fused mass of ganglia.

The PNS includes nerves emerging from brain and the ganglia supplying to respective nearby parts of the body.

Invertebrate brain, in fact, acts as relay centre between receptors and effectors. Hence, chopping the head of an invertebrate has little effect on movement, whereas in vertebrates the brain controls all movement of the body.